The present invention relates to a laser scan based recording apparatus for use with laser printers and the like to record information by causing laser light to scan the photosensitive surface of a photoreceptor with a rotating polygonal mirror. The invention relates specifically to an apparatus for adjusting the optical intensity of laser light being applied to the photosensitive surface of the photoreceptor.
Laser scan based recording apparatus of the type contemplated by the invention are adapted to be such that laser light emitted from a laser light source is projected on a reflecting surface of a rotatably driven polygonal mirror. As the polygonal mirror revolves, the direction of the laser light reflected from the reflecting surfaces of the polygonal mirror is changed so as to scan the photosensitive surface of a photoreceptor with the reflected light, thereby drawing an image in lines. In order to attain a uniform image density, the optical intensity of the laser light issuing from the laser light source (the intensity is hereunder referred to as "laser power") must be controlled at a specified level. To this and, the intensity of the laser light issuing from the laser light source is detected and the laser power is controlled on the basis of the detected value. This technique is generally called "APC (automatic power control)" and its operational concept is shown in FIG. 4. The optical intensity of laser light emitted from a laser diode LD in a laser light source 1 is detected with a monitoring photodiode PD. The resulting detection current Im is converted to voltage in a I/V (current/voltage) converter 11, followed by comparison with a reference voltage Vref in a comparator 13. The resulting comparison output Vo is held in a sample and hold circuit 14, from which it is fed into a laser drive circuit 15 capable of V/I (voltage/current) conversion. In the laser drive circuit 15, the current to drive the laser diode LD is controlled such that the laser power is controlled to a specified value.
A problem with this approach of controlling the laser power to a specified level is that if the intensity of laser light varies due to various factors that occur in the optical path from the laser light source to the photosensitive surface, it becomes difficult to ensure the desired uniform image density. One such factor is the variation in the reflectance of laser light from the individual reflecting surfaces of the polygonal mirror 4. If different reflecting surfaces produce different reflectances of laser light, the intensity of the laser light reflected form the successive reflecting faces also varies, causing differences in the densities of the scan lines created by the laser light reflected from the successive reflecting surfaces. The reflecting surfaces of the polygonal mirror are initially designed to reflect the incident light at the same reflectance. However, as the polygonal mirror revolves at high speed, dust and other particles in air collect on the reflecting surfaces. Because of this, and other reasons, the reflectance of laser light from each reflecting surface will vary over time, eventually causing variations of about 3 to 4% in the reflectances of laser light from the respective reflecting surfaces. Such variations in reflectance are not a big problem in binary level recording which forms an image by turning on and off the laser light. On the other hand, with color printers or the like which require the production of a halftone image, density control is necessary for providing at least 256 levels of contrast. To this need, the variation in the reflectance of laser light should not exceed 1%.
In order to deal with the problem of variation in the reflectance of laser light from each reflecting surface of the polygonal mirror, the detection of the intensity of laser light in the practice of APC may be performed in a position downstream of the polygonal mirror, preferably in the neighborhood of the photosensitive surface of the photoreceptor. In this way, the optical intensity of the laser light actually reflected by the polygonal mirror is detected and by controlling the laser power on the basis of the detection output. Uniformity can then be assured in the profile of laser light intensity on the photosensitive surface. For example, Unexamined Published Japanese Patent Application Nos. 37029/1988 and 162013/1994 propose a beam recording apparatus which detects the intensity of laser light just before it scans the photosensitive surface of a photoreceptor drum, and controls the laser power on the basis of the detected value of laser light intensity. More specifically, the optical intensity output of laser light is detected with a photosensor provided in the neighborhood of the photoreceptor drum. Its peak is held and fed back to an APC circuit, which uses the thus held sensor output to control the laser light modulator, as well as the semiconductor laser generator serving as the laser light source.
According to the technique just described above, the laser power control is solely based on the laser light intensity detected with the photosensor provided just before the photoreceptor drum. This occasionally causes the following problem. Prior to the projection of laser light on the photosensor as in the initial sate or if the laser light fails to be projected on the photosensor due, for example, to a vertical offset in laser scanning, the output of the photosensor is almost zero, so the APC circuit will control the laser power toward a maximum level. This causes the laser light source and the optical modulator to operate with a maximum level of laser power, as is typically the case of failure of the laser light to be projected on the photosensor. If the laser light source is continuously driven to produce a maximum power, the semiconductor laser generator may potentially break due to the overdrive. In the other case, the repeated variation in laser power from the maximum to a specified level eventually shortens the life of the semiconductor laser generator.